During use of any gas diffuser that is operated while submerged in a body of liquid that may contain suspended solids, such as waste water, eventual clogging of all outlet openings below a certain size in the walls of the gas plenum chamber of the diffuser is unavoidable because of dirt in the gas (such as air) that is pumped through the diffuser, the settling out of solid particles of various types that are suspended in the liquid, and the formation of deposits of iron or calcium compounds or other similar compounds. If such a diffuser is used under ordinary conditions of use for a long enough period of time, experience has shown that in some liquids any gas outlet openings that produce gas bubbles that are approximately 1/2" or smaller in diameter are likely to become completely clogged.
Gas diffusers are known that address this problem by providing, in addition to smaller openings, some alternative gas outlet openings that are large enough that they will never become clogged under any likely conditions of use. Thus, gas diffusers for operating in a body of liquid are known that include two groups of gas outlet openings from the gas plenum chamber, the first group being of smaller size and located at a higher level than the second group, with the openings in the second group being horizontal slots large enough that even if the openings in the first group become completely clogged after a long period of use, the larger slots will remain open indefinitely (under any ordinary conditions of use) no matter how long the diffuser remains submerged in the body of liquid into which gas is being introduced. The gas diffuser disclosed in Thayer U.S. Pat. No. 3,424,443 issued Jan. 28, 1969 is one example of such a device.
In the device of the Thayer patent, gas is introduced at the top of the plenum chamber to displace liquid that initially flowed into the plenum from the body of liquid in which the plenum chamber is submerged. After enough gas has accumulated under the impermeable top wall and impermeable upper wall portions of the plenum chamber to lower the surface of the liquid in the chamber to the level of the smaller outlet openings in the side walls of the chamber, the gas flows out through these smaller openings. So long as these smaller openings do not close up completely from the clogging that (as explained above) unavoidably occurs during extended periods of diffuser use, some gas is permitted to exit through the side walls.
The resulting bubbles grow smaller as the openings gradually become more and more clogged. In addition, the Thayer device attempts to decrease the size of the bubbles from the side wall openings by constructing the bottom wall of the gas plenum chamber in a special configuration that is intended to give the upwardly directed liquid currents (resulting from the rising gas bubbles after they are emitted from the side wall openings) a shearing effect as they move upward past the side walls of the chamber.
After the side wall openings of the gas plenum chamber in a device such as the Thayer diffuser have become completely clogged, the pressurized gas enclosed within the plenum beneath the impermeable top wall--and now the impermeable side walls--pushes the surface of the liquid in the lower portions of the plenum chamber down to the level of the horizontal "emergency air release slots" at the bottom of the chamber (col. 2, lines 25-26).
The Thayer invention thus achieves a gas diffuser having an indefinite life through utilization of a gas plenum chamber having an impermeable top and impermeable upper wall portions to produce gas flow on a consecutive basis from the same gas plenum--initially through first, smaller gas outlet openings at a level part way down the side walls of the plenum chamber and after those openings become completely clogged, through second, much larger gas outlet openings in the form of horizontal slots at the bottom of the plenum chamber (col. 1, line 70 to col. 2, line 2; col. 3, lines 43-47).
U.S. Pat. No. 3,608,834 to MacLaren discloses an air diffuser similar to the Thayer device in that it utilizes openings in the side walls of the plenum chamber and notches at the open bottom of the chamber. The plenum chamber of the MacLaren diffuser, like that of the Thayer diffuser, has not only impermeable upper wall portions but also an impermeable top wall (except for an inlet valve that opens to admit air into the plenum chamber during operation of the device), and thus the two diffusers operate basically in much the same way.
In the Thayer and MacLaren difusers, although both air ports 23 and openings 57 spaced above the bottoms of the respective side walls of the diffusers are smaller than longitudinal air escape slots 27 and notches 46 or 56 at the bottom of the side walls, they are still not small enough to produce fine gas bubbles as defined below in this specification. Furthermore, even if they were small enough, the bubbles emitted from Thayer ports 23 and MacLaren upper openings 57 are formed only in single narrow zones along the perimeters of the respective gas diffusers, which zones are defined by the side walls of the plenum chambers, instead of across the entire porous area of the top wall of the diffuser, as is the case with the present invention.